Polyesters that are semicrystalline, particularly poly(ethylene terephthalate), PET, are used extensively in many applications that require good solvent resistance and good properties at elevated temperatures. They are ordinarily processed by injection molding, but there are many components of automobiles and other systems wherein such parts are hollow and to manufacture these by injection molding is very difficult and expensive. Many such parts can conceivably be made by blow molding provided the polymer system has adequate melt strength and melt viscosity and yields smooth high quality surfaces in the blow molded parts. Unfortunately, polyesters commercially available for injection molding and extrusion have melt viscosities which are too low to make them suitable for extrusion blow molding. It would be desirable to have blow moldable polyester compositions which provide moldings having smooth surfaces made from commercial injection moldable and extrusion grades of polyesters.
The addition of conventional di- and polyepoxides and, more recently, the addition of ethylene copolymers containing glycidyl groups have been suggested for increasing the melt strength and viscosity of polyesters, but are not suitable for blow molding large objects having smooth surfaces and having complex cross-sections such as automobile parts.
Further improvements in melt strength and melt viscosity have been provided by compositions which in addition to the ethylene copolymers containing glycidyl groups use small amounts of catalytic cations which may be introduced in the form of a zinc ionomer. Unfortunately it has been found that these catalyzed compositions may form small lumps when the compositions are exposed to processing temperatures for an extended period of time. Such prolonged exposure is not unusual in commercial blow molding operations where a substantial proportion of the resin must be recycled as regrinds. The presence of these lumps results in molded objects having surface blemishes or surface irregularities.
Thus a need still exists for polyester compositions, particularly for PET-based compositions, which have sufficient melt strength and viscosity to permit extrusion blow molding of large and complex objects which at the same time exhibit uniform, smooth surfaces.